1. Field of the Invention
This invention relates to drill bits. Aspects of the invention relate to both fixed and expandable drill bits.
2. Description of the Related Art
Deep bores, for example, as utilized to access subsurface hydrocarbon-bearing formations, are conventionally drilled using drill bits mounted on the end of a string of drill pipe, the drill pipe being rotated from surface. It is also known to drill bores using drill bits driven by downhole motors, and to mount drill bits on relatively flexible coil tubing. There are currently two main drill bit forms in common usage, that is roller cone bits and fixed cutter bits, the former cutting primarily by a crushing action and the latter relying primarily upon a shearing action.
In drilling deep bores, it is common to drill a section of bore and then line or case the bore before drilling further. Using a conventional fixed diameter bit, any subsequent drilling produces a relatively small diameter bore; the drill bit must be able to pass down through the cased section of bore, and must therefore be of smaller diameter than the casing. Such a loss of diameter may be minimized by, for example, using a bi-centre bit, or by providing an expandable bit. However, use of a bi-centre bit generally must be preceded by drilling a short pilot hole which must then be under reamed to accommodate the bit. Also, although numerous expandable bits have been proposed, these are of complex construction and the applicant is unaware of any such bit in current commercial usage.
It is among the objectives of embodiments of the present invention to obviate or mitigate such disadvantages, and one aspect of the invention relates to an improved expandable drill bit arrangement.